1. Introduction
This invention relates to an improved method of electrodepositing photosensitive materials in which the coating bath contains an electrodialysis cell as a means of controlling the composition of the bath. In particular, the system provides a means of removing undesirable components from a photoresists formulation bath while allowing continuous monitoring of the conductivity of the system.
2. Discussion of Prior Art
Electrocoating is a term commonly used in the automotive industry for the combined process of electrolytic and electrophoretic deposition of paints onto conductive parts. The electrocoating paint bath usually contains a resin system, a cross-linker, pigments, solvents, additives and a solubilizing agent to render the other ingredients water soluble or dispersible. In anodic electrocoating systems, the solubilizer is a base and in the cathodic process it is an acid. In order to coat articles, an electric current is applied to a bath to cause the particles to migrate towards the electrode of opposite charge. In the anodic system, the solubilizer migrates to the cathode, and the charged colloidal particles migrate to the anode. The cathodic process is the opposite. The cathode becomes the workpiece or coated object. Under an applied voltage, the solubilizer, usually an organic acid, migrates to the anode, which is constructed of an inert material. The solubilizer reacts with hydrogen ions generated from the electrolysis of water to form the free acid. The acid may be used for solubilizing the incoming replenishment, or it can be flushed out of the system. At the cathode, the electrolysis of water forms an alkaline boundary layer which neutralizes and coagulates the incoming colloidal resin particles depositing them as a water-insoluble film.
The composition of the paint bath differs from the composition of the deposited film because various components of the bath migrate at different rates. If this is not accounted for in the replenishment formula, a change in bath composition will occur. If the solubilizer content is allowed to increase in the bath, poor film quality and possible resolubilization of the deposited film can occur. Conversely, if the solubilizer content is allowed to drop, insolubility of the bath can occur. Therefore, it is desirable to incorporate some means of controlling the solubilizing agent.
Common methods used to control the solubilizer content include electrodialysis and ultrafiltration. Electrodialysis utilizes a semi-permeable membrane box fitted around the tank electrode. The box captures free solubilizer during the coating process and removes it from the system usually by flushing it to drain. This procedure allows the addition of a completely solubilized and water-soluble system as replenishment material. Major problems with this approach are the very high cost of installation, maintenance of the membrane box or boxes, waste of solubilizer and water pollution. Ultrafiltration is used to make minor adjustments in the solubilizer content. Complete control of the solubilizer is rarely attempted with this method, due to the need for extremely large volumes of makeup deionized water. The ultrafiltration process utilizes a semi-permeable membrane which readily allows small molecules such as water, solvent, solubilizer and ionic salts to pass through while retaining the larger paint particles or micelles. Liquid passing through is called permeate. By using a closed-loop ultrafiltration system, the permeate generated during the filtration process can be used to rinse the coated articles and recover dragged-out paint from the film. The permeate could then be returned to the bath with the paint particles. This system solved the problem of wasted paint particles and provided a means of controlling the tank for excess conductivity.
U.S. Pat. No. 3,304,250 discloses a process utilizing electrodialysis to control the bath composition in an electrocoating process. In the practice of the invention, a dialysis membrane, which is permeable to water and water soluble resin dispersal agents, is placed in the bath between the article to be coated and the tank electrode. The membrane is impermeable to the resin and therefore creates a coating zone which contains the resin dispersion and a zone which is free of resin and contains the water soluble resin dispersant, thereby providing a means for controlling the level of dispersant in the bath. Replenishment solutions may then be added to the coating zone to control the composition.
Although electrodialysis within the bath provides excellent control of solubilizer, it is very expensive and inconvenient. It is often necessary to use more than one electrodialysis unit in the bath and the membranes are susceptible to damage by the articles being coated. The membranes may also become clogged with paint rendering them inactive.
U.S. Pat. No. 4,775,478 discloses a process for removing acid from cathodic electrocoating baths. The patent discloses a means of controlling the bath composition by removing a portion of the bath and subjecting it to ultrafiltration. The solution to be ultrafiltered is brought, under pressure, into contact with a filtration membrane arranged on a porous carrier in a cell. The contents of the ultrafiltration cell are stirred in order to prevent accumulation of the retained material on the membrane surface. Ultrafiltrate is formed continuously and is collected until the retained solution in the cell has reached the desired concentration. Part or all of the ultrafiltrate is then fed into an exchange cell along with an aqueous solution of a base to remove excess acid by dialysis. The dialysis process is carried out without the use of an electrical field.
Great Britain Patent No. 2 111 080 discloses a similar process for treatment of an electrodeposition bath combining ultrafiltration and electrodialysis. The ultrafiltration process is the same as described above but the next step subjects the ultrafiltrate to electrodialysis. The process involves use of an electromotive force to accelerate dialysis. A portion of the ultrafiltrate is passed to the electrodialysis unit where the solubilizer and undesirable contaminants are forced through a semi-permeable membrane and expelled from the system. The ultrafiltrate which has been subjected to electrodialysis is then returned to the electrodeposition tank or may be used first to rinse coated articles prior to return to the tank.
All of the above procedures pertain to electrocoating which is a term of art used in the automotive industry. The photoresists industry has begun using a similar process called electrodeposition which refers to depositing photoresists coatings onto conductive substrates under an applied voltage. The process is useful in the manufacture of electronic devices such as printed circuit boards. Photoresists coatings are deposited so that they readily adhere to the substrate as a continuous film, yet unlike paint coatings, they must be easily removed from the substrate during subsequent processing steps. Thus, control of the bath composition is critical to the quality of the final product obtained.
In the electrodeposition of photoresists coatings, several factors effect the quality of the deposited film. These factors include temperature, applied voltage, and conductivity of the bath. Without a means of controlling and monitoring the bath composition, the photoresists coating may display poor film quality and poor adhesion to the substrate. Bath stability would also be negatively effected if process conditions are not controlled.
Photoresists may be deposited on conductive substrates both cathodically and anodically. In the process of cathodic electrodeposition of photoresists, the positively charged micelles of the photoresists migrate to the conductive substrate where they are deposited as a uniform film. The negatively charged counter-ion migrates to the electrode of opposite charge where it is neutralized back to acid by the hydrolysis of water. Without some means of removing it, the acid concentration will continue to build up in the photoresists bath increasing conductivity of the bath and causing the emulsified photoresists formulation to degrade. The buildup of this counter-ion also contributes to loss of the ability to control the film thickness and in the deposited film being badly pin-holed. Both of these defects account for failures in the performance of the printed circuit board.
If the acid is isolated into a confined area with the anode the conductivity of the system must be monitored to prevent corrosion of the anode. Monitoring of the conductivity of the system may be done by placing a conductivity probe into the isolated area containing the acid which has been removed, by some means, from the emulsion. Typically, the conductivity is measured by placing a conductivity probe into the isolated area and adjustments are made to the system as needed.